A high-spin Fe(II)/low-spin Fe(III) redox couple featuring the hydro[tris(4-chloro-3,5-dimethyl-pyrazolyl)]borate ligand: Synthesis, spectroscopic and X-ray crystallographic characterization

The white homoleptic high-spin iron(II) complexes Fe[Tp(Me2,4Cl)](2) (1) was isolated in quantitative yield from reaction mixtures containing 1 equiv of FeCl2(THF) 1.5 and 2 equiv of K[Tp(Me2,4Cl)] (Tp(Me2,4Cl) = hydrotris[(4-chloro-3,5-dimethyl-pyrazolyl)]borate). Its purple low-spin iron(III) counterparts 1[O3SCF3] and 1[PF6] were synthesized and isolated in 85% yields upon treatment of 1 with 1 equiv of silver trifiate and silver hexafluorophosphate, respectively. The three paramagnetic compounds are air and thermally stable as solids and in solution; they were characterized by elemental analyses, IR, magnetic susceptibility measurements, H-1 NMR, and Mossbauer spectroscopy. In addition, 1[PF6] was authenticated by a single-crystal X-ray diffraction. The two scorpionate ligands are kappa(3)-N,N',N '' ligated to the central Fe-III ion, forming an almost perfect FeN6 octahedron with an average Fe-N bond distance of 1.9551(18) angstrom. In addition, complex 1 which oxidizes reversibly at E-1/2 = 0.483 V/SCE (Delta E-p = 94 mV), remains high-spin (S = 2) when the temperature is lowered to 2 K. (C) 2009 Elsevier B.V. All rights reserved.